CDMA (code division multiple access) is a wireless communication technique which employs spread-spectrum single-sideband technology. Commercial implementations of CDMA technology typically take the form of a digital cellular phone and/or data communication service in which a code is assigned to all speech or user data bits, scrambled transmission of the encoded speech is wirelessly transmitted and the speech (or data) reassembled to its original format upon delivery to the receiver.
In a CDMA system, each device is given a unique sequence, called a pseudo-random code. This sequence identifies the device. For example, if device A has sequence-A and device B has sequence-B, a receiver receiving data from device-A uses sequence-A to decode the wanted information. The receiver receives the energy (power) being transmitted by device-A and disregards the energy transmitted by device-B. Because device-A and device-B are isolated by having different codes, they can share the same carrier frequency, partially eliminating the frequency reuse problem encountered by previous cellular communication technologies.
CDMA is an interference-limited system. Although it has a soft capacity limit, each device is a noise source on the shared channel and the noise contributed by devices accumulates. This creates a practical limit to how many devices the system will sustain. Mobile devices which transmit excessive power increase interference to other mobile devices. For CDMA, precise power control of mobile devices is important in maximizing the system's capacity and increasing the battery life of the devices. The goal is to keep each device at the absolute minimum power level required to ensure acceptable service quality.
Thus, in CDMA mobile device-based systems, closed loop power control is employed to maintain acceptable service quality for all mobile station receivers at the base station. To accomplish this power control, the receiver typically performs an estimation of the received signal and noise power levels, determines a signal to noise ratio (SNR) and compares the SNR with an SNR threshold. If the received SNR is less than the threshold, a power-up command, typically in the form of a single bit, is embedded in the forward link and transmitted from the base station to the mobile device, requesting an increase in transmit power. Otherwise, a power-down command is sent to the mobile device to lower its transmit power.
This power control command is embedded in the forward CDMA channel in the form of reverse link power control bits. In general, reverse link power control methods are known. For example, reverse link power control bits are implemented in some cases in pseudo-random positions in each 1.25 ms interval which employs power control (power control group or PCG), or 16 times per frame in standards which employ power control such as the Air Interface Standard IS-95 and the CDMA2000 standard. Each power control bit is interpreted as a command to raise or lower power by a predetermined increment. Each base station makes power control decisions independently for each mobile device. The mobile device demodulates the power control bits and raises or lowers its transmit power accordingly. As stated above, the goal of power control is to maintain the reverse channel (mobile devices to base station) transmit power at the lowest possible level to achieve a given error rate performance.
One approach common to CDMA systems is the use of a Rake receiver for combining information obtained from several resolvable multi-path components in order to combine an enhanced signal with a high voice/data quality. The Rake receiver includes a bank of correlators, also referred to as demodulators, each of which correlate to a particular multi-path component of the desired signal. The correlator outputs, commonly referred to as the Rake channels or fingers, may be rated according to their relative strengths and summed to obtain a final signal estimate.
Obviously, it is important to accurately obtain an estimation of the received SNR for proper power control. In the current implementation of the Air Interface Standard IS-95 reverse channel, there is only one channel per user link. The reverse channel uses 64-ary orthogonal modulation symbols, specifically Walsh functions of order 64. The SNR estimation is performed by processing the Walsh spectrum of the Rake output. There are 64 elements in the orthogonal demodulation output of the Rake receiver. By regarding the largest element as the signal contribution and the rest as being caused by noise and interference, one can obtain an SNR estimation at the Rake receiver output. In the CDMA2000 Air Interface Standard, each user has a continuous pilot channel which can be used for SNR estimation and up to four reverse sub-channels, each having different rates and power levels. It is therefore important to be able to accurately measure the received signal power level and received noise level in order to accurately determine the SNR for subsequent power control bit determination.
Further, standards such as the CDMA2000 Air Interface Standard provide for supplemental channels which are sometimes used and sometimes not used. This can lead to inaccurate measurement of the total channel power, particularly in the case of estimating the noise power level. It is therefore desirable to have a method which provides for an accurate and reliable noise power level determination, regardless of whether supplemental channels are employed.
Accurate estimation becomes even more important when dealing with wireless high speed data communication (e.g. 100 Kbits/second and faster). Conventional methods for estimating the received noise power include measuring the total energy received by the base station. As such, the estimated noise power is based on both the signal energy and noise energy. These methods do not provide sufficient accuracy when the mobile device signal includes high speed data. Conventional methods for estimating the pilot signal energy include despreading the received signal and coherently accumulating the despreaded received signal over a certain interval, for example, 384 chips, and then non-coherently accumulating over one power control group. However, the phase disturbances due to channel fading may degrade estimation accuracy. As such, selecting a power control group referenced to the user frame as the interval selected for estimation of signal power may not be close enough to the position of the reverse power control bit embedded in the forward CDMA channel to provide an accurate estimate. This is the case because the delay from the time the signal power is estimated to the time the power control bit is embedded in the transmit signal may be so great as to not accurately represent a change in the quality of the channel. This can result from movement of the mobile device and/or the surroundings.
As such, it is desirable to reduce the power control delay in a manner such that the selected interval for estimating the signal power is as close as possible to the position of the reverse power control bit to maximize the accuracy of the power control bit determination. It is further desirable to have a method for estimating the received signal energy which employs a correlation process which significantly reduces the phase disturbance due to fading.
Conventional methods use one complete power control group signal to estimate signal power. If the wireless device is far from the base station, the base station may not be able to send the power control bit on the forward link in the next PCG because the base station may not be able to complete processing in a manner which provides enough time to insert and transmit the power control bit in the outbound power control group position. As such, in accordance with known standards, the power control bit must be inserted in the second power control group following the corresponding reverse link power control group in which the signal power was estimated. These conventional methods provide for the possible distance differences between wireless devices and their corresponding base stations by incorporating undesirable delay. It is desirable to have an arrangement which allows the power control bit determination to be made as temporally close to the point at which the power control bit must be inserted as possible.